The invention relates to an electric high-pressure discharge lamp for use as a motor vehicle headlamp, comprising
a discharge vessel having an axis and provided with a discharge space between a first and a second end portion, which end portions comprise respective seals adjoining said discharge space, in which a pair of electrodes and a gas filling are present, current supply conductors of the pair of electrodes extending to the exterior through respective seals.
a holder body of insulating material in which the first end portion of the discharge vessel is indetachably secured, which holder body is provided with electrical contacts which are connected to respective current supply conductors.
Such a lamp is known from EP 030941-A, which corresponds to U.S. Pat. No. 4,868,456.
A disadvantage of the use of a discharge lamp as a motor vehicle headlamp is that without special measures such a lamp does not yield sufficient light instantaneously upon switching on of a supply source. If within a very short period after ignition of the lamp a considerable fraction of the luminous flux is to be obtained, which is achieved in the stable operating condition of the lamp, the lamp may be operated at an extra high current before that stable condition is achieved.
The time required for obtaining the considerable fraction of the luminous flux, however, depends on, for example, the temperature of the lamp during ignition. If the lamp is operated at an extra high current for the same period under all circunstances, this period may sometimes be too short, sometimes too long. If the period is too short, the considerable fraction of the luminous flux is not obtained quickly enough. If the period is too long, the result is an overshoot. This is disadvantageous because the excessive luminous flux of the lamp may cause glare and because the useful flie of the lamp or its supply unit may be shortened.
U.S. Pat. No. 3,681,654 discloses how a pipe is aimed at a low pressure discharge lamp, between its electrodes, guiding the received light to a photosensitive transistor of a control circuit in order to control the luminous flux of the lamp.
U.S. Pat. No. 3,483,428 discloses a photosensitive variable-impedance element, such as a photosensitive resistor, which is aimed at a discharge lamp in order to control the power consumed by this lamp.
DE 15 63 971 C3 discloses how a photoelectrical or heat-sensitive device is applied against a low-pressure discharge lamp laterally of the discharge path, which device is connected to a control circuit.
The known means are not suitable for controlling the power consumed by a lamp of the type described in the opening paragraph. A heat-sensitive device is not eligible since it is of essential importance for controlling lamp power whether the lamp has a certain temperature as a result of a recently extinguished discharge or as a result of a recently ignited discharge.
A lamp designed for use in an optical system comprising a reflector and a lens in order to form a profiled light beam, possibly containing sharp light/dark cut-offs, as in a passing beam, has a very small discharge arc with a length of a few mm, for example 5 mm or less, and a diameter of approximately 1 mm. A light sensor positioned against the discharge vessel would cut off a too large solid angle and severely affect the light beam formed.
Furthermore, a motor vehicle headlamp is used in surroundings comprising other light sources, for example headlamps of other motor vehicles coming from the opposite direction, which can shine into the headlamp concerned. A sensor mounted in the reflector of a motor vehicle headlamp can thus react to light coming from outside the headlamp and control the supply of the discharge lamp incorrectly as a result. If the discharge lamp is mounted interchangeably in a motor vehicle headlamp, pollution of a sensor mounted in the reflector is possible, so that again an incorrect signal is given. Moreover, such a headlamp has the disadvantage that, if the discharge lamp is replaced with a discharge lamp of a different make, the lamp and the sensor may be badly attuned to one another.